1. Field of the Invention
The present invention relates to a can type secondary battery, and more particularly to a sealing structure for an electrolyte injection hole of a can type secondary battery.
2. Description of the Related Art
Secondary batteries are rechargeable batteries which can be fabricated in a compact size with large capacity. Among various secondary batteries, nickel-metal hydride (Ni—MH) batteries and lithium-ion (Li-ion) batteries have been developed and used as can type secondary batteries. Secondary batteries may be classified into various types, depending on an electrolyte used. Such electrolytes may be, for example, a liquid electrolyte, a solid polymer electrolyte or a gel-phase electrolyte.
In the case of a lithium secondary battery using a liquid electrolyte, a non-aqueous type liquid electrolyte must be used due to a reaction between lithium and water (H2O). Since the lithium secondary battery uses the non-aqueous type liquid electrolyte, the lithium secondary battery is not subject to decomposition voltage of water during a charging operation thereof, so that the lithium secondary battery has relatively high battery voltage.
Liquid electrolytes are composed of lithium salts dissociated in an organic solvent. The organic solvent may include ethylene carbonate, propylene carbonate, carbonate containing an alkyl group, or organic compounds similar to the above components.
A lithium secondary battery using a solid electrolyte may not create leakage of the solid electrolyte. However, similarly to a general chemical battery, it is desirable for a can type lithium ion secondary battery using the liquid electrolyte to prevent the liquid electrolyte from being leaked. In particular, since the lithium ion secondary battery may be used as a power source for a portable telephone, a computer, a PDA, and a camcorder, which are expensive electronic appliances, the leakage of the liquid electrolyte is a problem to be solved.
Typically, leakage of the liquid electrolyte is created in a welding section between a can and a cap assembly and an electrolyte injection hole of the cap assembly in the can type secondary battery.
FIG. 1 is a partial sectional view showing an upper portion of a can type secondary battery including an electrolyte injection hole 112 of a cap plate 110 and a plug.
Referring to FIG. 1, after the electrode assembly 12 has been inserted into a can 11, an opening of the can 11 is sealed by means of a cap assembly 100. The cap assembly 100 is bonded to the can 11 by welding such that the opening of the can 11 is covered with the cap assembly. The electrolyte injection hole 112 is formed in the cap plate 110 of the cap assembly 100. After the cap assembly 100 is welded to the can 11, an electrolyte is injected into the can 11 through the electrolyte injection hole 112. Then, a plug 160 in the form of a ball is press-fitted into the electrolyte injection hole 112 so as to seal the electrolyte injection hole 112. The plug 160 is press-fitted into the electrolyte injection hole 112 formed at one side of the cap plate 110 and is welded to the cap plate 110. Welding the plug 160 to the cap plate 110 is necessary because otherwise the electrolyte may leak through a fine gap formed between the plug 160 and the cap plate 110 even if the plug 160 is mechanically press-fitted into the electrolyte injection hole 112.
The cap plate 110 and the ball forming the plug 160 are typically made from aluminum. Since aluminum has superior electrical and thermal conductive properties, laser welding is typically used for welding the plug 160 to the cap plate 110. When a laser beam is irradiated onto a welding section formed at an edge of the plug 160, the plug 160 and an inner portion of the electrolyte injection hole 112 formed in the cap plate 110 are partially welded, so that the plug 160 is welded to the cap plate 110.
Recently, a can has been made having a reduced size for a lighter weight and higher battery capacity. Accordingly, a cap plate having a thickness less than 1 mm has been recently fabricated. If the thickness of the cap plate is reduced, mechanical strength of the cap plate is lowered and the possibility of deformation of the cap plate caused by external force is increased. In particular, in the case of a can type secondary battery in which a safety vent is formed in the cap plate rather than in a lower portion of the can, if the safety vent is positioned adjacent to a processing area of the cap plate, the cap plate may be extremely deformed by external forces from processing the cap plate.
If the cap plate is easily deformed by external forces applied to it during a manufacturing process, a crack may form in the welding section and certain processing steps, such as welding, may not be easily carried out. Thus, the electrolyte may leak as a result of welding failure.
FIG. 2 is a partial sectional view showing the problem created in the vicinity of an electrolyte injection hole when the can is sealed by means of an aluminum ball press-fitted into the electrolyte injection hole. FIG. 3 is a partial sectional view showing a problem when welding work is carried out with respect to a sealed section as shown in FIG. 2.
Referring to FIGS. 2 and 3, a predetermined portion of the cap plate 110 adjacent to the electrolyte injection hole is depressed as the aluminum ball is press-fitted into the electrolyte injection hole. In addition, the aluminum ball forming a plug 160′ is not sufficiently inserted into the electrolyte injection hole, and an upper portion of the aluminum ball is upwardly protruded from an upper surface of the cap plate 110. In addition, a lower portion of the electrolyte injection hole formed in the cap plate 110 becomes wider so that a predetermined portion of the aluminum ball inserted into the electrolyte injection hole. In other words, an outer surface of the plug 160′ does not make close contact with an inner wall of the electrolyte injection hole, but rather, only makes contact with an inlet portion of the electrolyte injection hole. Accordingly, the sealing function of the plug 160′ for the electrolyte injection hole may deteriorate. As a result, the electrolyte contained in the can may flow up to the inlet portion of the electrolyte injection hole and a gap may be formed between the plug 160′ and the electrolyte injection hole at the inlet portion of the electrolyte injection hole. In particular, if pressing force applied to the aluminum ball causes the deformation of the battery, leakage of the electrolyte may occur.
In addition, although the electrolyte will not leak to an upper surface of the cap plate 110, the gap formed between the plug 160′ and the electrolyte injection hole may fill up with the electrolyte. If welding work is then carried out with respect to the welding section formed between the plug 160′ and the cap plate 110 forming the inner wall of the electrolyte injection hole, the weld may be less reliable if the electrolyte contaminates the welding section formed between the plug 160′ and the electrolyte injection hole. In addition, as shown in FIG. 3, an impurity area 162 called “spatter” is formed in the contaminated welding section which may allow electrolyte to be leaked through the impurity area or a pinhole formed in the welding section after the impurity area is removed from the welding section. Otherwise, external humidity or oxygen may penetrate into the can through the pinhole, thereby causing swelling. Therefore a need exists for a can type secondary battery capable of reliably sealing an electrolyte injection hole.